I thought I would give an update to help others with the usefull information I have found on the subject.
First. Although many use plain distilled water in their cutters, even though they don't need any sort of "freeze" protection, plain water without any anti fungal additives could cause bacterial growth on the inside of the tube surface causing a mild reduction in cooling performance.
Now onto coolants. especially for those of us in colder climates.
Many suggest using automotive "green" antifreeze, which is a better alternative, however there is one thing to keep in mind, especially with automotive "green" coolants is the potential issues of the added "sodium silicate" in the coolant to help as a stop leak. Although this could be argued that it could/could not affect a laser cooling system. It is something to keep in mind depending on how your cooling system is designed.
The optimal "anti-freeze" protection coolant would be one based on an ethylene glycol chemistry. There is also a propylene glycol chemistry coolant that is more environmentally friendly, however has a slight reduction in cooling capacity/thermal conductivity compared to a ethylene system. This could be a minor/ insignificant effect on a laser tube cooling system (approx. 1-5% reduction).
If its available, tig welder cooling system coolant is a good alternative, many are clear (after looking at my W4, the coolant loop does not affect optics at all), they are usually ethylene glycol based, and are tested to have a low conductivity with HV.
After looking at some available options to me, I have come to the conclusion I will be going with a Zertex G-05 coolant with a 50/50 mix with distilled water. The MSDS of G-05 lists a 90%+ concentration of Ethlyene Glycol, 3% mix of Diethlyene glycol, 3% mix of Sodium Benzoate (anti fungal additive and corrosion inhibitor), and a 3% mix of Disodium Tetraborate ( acts as a anti fungal and corrosion inhibitor)
I would of like to have used plumbing antifreeze it does contain Propylene Glycol, its main base however is Ethanol, which is flammable. Unless you can guarantee a perfectly lifetime leak free system, there is a mild risk with leaks and the HV. Plus in a closed loop system, it can evaporate and pressurize the coolant loop during temperature rise changes. I have used RV antifreeze in my Tig setup without issues, I will eventually change to G-05 in that system as well.